1. Field of the Invention
The present invention relates to an induction sensor for sensing the displacement of a movable body by electromagnetic induction.
2. Description of the Related Art
An induction sensor has customarily been used to sense, based on electromagnetic induction, the displacement of a body movable in a reciprocating motion in a predetermined direction, e.g., a printing mechanism included in a printer. The induction sensor is made up of a slider and a scaler each having flat coils printed on one surface thereof. The scaler, or slider body, is affixed to a printer or similar apparatus while the slider is mounted on a printing mechanism or similar movable body. It is a common practice to implement both the scaler and the slider with coil patterns printed on respective circuit boards. The substrate for the scaler usually has two coil patterns thereon which alternate with each other in the direction in which a displacement should be sensed. Sinusoidal currents different in phase by 90 degrees are respectively fed to the two coil patterns. The resulting magnetic field is sensed by the slider as sinusoidal waves whose phase change in accordance with a displacement.
Japanese Patent Laid-Open Publication No. 2-275314 discloses an induction sensor including a scaler which has two coil patterns printed on a substrate and each having two or more round-trip passes in order to reduce the ratio of the wasteful areas to the entire area of the substrate. This successfully enhances the dense arrangement of coil patterns in the displacement sensing direction and thereby increases the sensing accuracy. Japanese Patent Laid-Open Publication No. 5-135236 teaches an induction sensor including a scaler which has two coil patterns printed on a substrate and each having extra portions. The extra portions provide the coil patterns with the same length and, therefore, the same impedance, thereby promoting accurate sensing.
The conventional induction sensors described above have a problem that when the slider and scaler, i.e., substrates thereof are to be mounted to the respective positions, there must be finely adjusted the gap length, azimuth angle and so forth of the slider and scaler. Another problem is that when coil patterns are printed on a substrate, wasteful regions between the patterns cannot be eliminated. In addition, when the magnetic field to be generated by the scaler is intensified to enhance the sensing accuracy of the slider, it is necessary to increase the sinusoidal currents to be fed to the coil patterns of the scaler.